Which oxygen sensor most likely faulty?

[AD2BU]

I have a 2002 Ford E-150 85,000 mis with the "service engine soon" light on. Checked the codes and I think this is a bad O2 sensor. Since I've been told there are three on this vehicle (1 on each side in front of the converter and 1 behind the converter), can someone tell me which one is most likely to fail first?

Thanks for the help.

[rhandwor]

My scanner states which sensor to check.